A modern integrated circuit (IC) must meet very stringent design and performance specifications. In many applications for communication devices, transmit and receive signals are carried using a differential signal. A differential signal is one that is represented by two complementary signals on different conductors, with the term “differential” representing the difference between the two complementary signals. A “differential pair” is a communication methodology that uses two separate conductors (i.e., the differential pair) to carry a differential communication signal. All differential signals also have what is referred to as a “common mode,” which represents the average of the two differential signals. One of the conductors carries a “true” or “positive” version of the differential communication signal while the other conductor carries the “complement” or “negative” version of the differential communication signal.
One of the design factors to be taken into account is the influence of signal cross-talk between two differential pairs of transmit and receive signal conductors. Signal cross-talk is the influence or intermingling of signal energy from one conductor to a nearby conductor. Cross-talk degrades the signal integrity and in extreme cases, can lead to errors in the communication channel. In a modern IC, the circuit device, also referred to as the “chip” is typically attached to a printed circuit board through the use of an intervening structure, sometimes referred to as an interposer or package. The circuit layout of the chip is typically a very dense two-dimensional array of conductors. The interposing structure acts as a mechanical and electrical interface to “spread out” the dense array of circuit contacts on the chip to a less dense array of contacts on the PCB. The chip is typically attached to the interposing structure using an array of fine pitch solder bumps. The interposing structure is typically attached to the PCB using an array of solder balls that is substantially less dense than the array of solder bumps that attach the chip to the interposing structure. The array of solder balls that attach the interposing structure to the PCB is referred to as a ball grid array (BGA) or a BGA pattern.
Previous attempts to minimize signal cross-talk between signals carried on differential pairs include assigning the differential pairs on the BGA layout with one or more power or ground conductors separating the two differential pairs so as to increase signal isolation and minimize crosstalk between the differential pairs. Unfortunately, this solution consumes valuable area on the PCB and may not be the most efficient use of the BGA pattern.
Therefore, it would be desirable to have a way to minimize differential signal cross-talk while maximizing the efficiency of a BGA layout.